The present invention relates to an electric dust collecting apparatus, and more particularly, to improvements in a charging section in a two-stage type of electric dust collecting apparatus consisting of a charging section and a dust collecting section, and the invention provides a charging device for dust or a type having a higher electric resistance than has been heretofore considered possible to charge.
Among the conventional dust collectors, a so-called two-stage type of electric dust collector is known, which comprises a charging section for mainly causing the dust to be charged and a dust collecting section for mainly collecting the charged dust.
In the above-mentioned prior art type of charging section, an electrode arrangement in which a group of discharge electrodes consisting of wires or barbed conductor rods are insulatively disposed on the middle plane in the space gap between adjacent ones of a group of opposite electrodes consisting of parallel planar plates arranged in parallel to each other at an equal distance, has been mostly employed. However, the particle charging device having such an electrode arrangement could not obviate the basic technical difficulties that, firstly, insufficiently charged particles are produced, owing to space unevenness of an ion flow, and secondly, when the electric resistance of the dust is extremely high and exceeds 10.sup.11 (.OMEGA./cm), the dust loses its electric charge owing to inverse ionization caused by an accumulated dust layer on the opposite electrodes, whereby the particles vent at the downstream end conveyed by the gas, and consequently, the charging effect was greatly degraded and the dust collecting efficiency as a whole was limited in practice. More particularly, with regard to the aforementioned second shortcoming, it is caused, as described above, by the fact that the electric resistance of the dust layer deposited and accumulated on the opposite electrode becomes extremely high, in fact, to such an extent that the electric field therein exceeds a field breakdown intensity. When this happens, a breakdown occurs within the dust layer producing inverse ionization points on the dust layer from which are fed ions of opposite polarity to the ions fed from the discharge electrode. As a result, these opposite polarity ions neutralize the particle charge. These particles, having lost their charge, are conveyed by the gas and enter the dust collecting section.
In view of the aforementioned shortcomings in the prior art, the inventors of the subject invention tried experiments employing cylindrical rod-shaped electrodes as opposite electrodes, but even in such a case, satisfactory results could not always be obtained in a charging device for an exhaust gas containing dust having a high electric resistance. This was because when a dust layer was formed on the surface of the rod, inverse ionization occurred in the case of a high electric current density.